Roller Grinding mills £ļ
1. General Outline
BEIJING POWER EQUIPMENT GROUP (BPEG) is the leading manufacturer
of mills in China. BPEG has been manufacturing mills since the end
of 1950's. They are therefore one of our oldest products.
Today an extensive range of them is available for a great variety
of industrial engineering and these mills are, in most instances,
designed to the customer's requirements. BPEG's product spectrum
includes, for instance, medium speed mills and low speed tube ball
mills and so on. For new plants and techniques, BPEG has frequently
been a pioneer (trend-setter) in the planning and developing of
the required powder making engineering.
2. Technical Milestones in Development and Design
Creativity and the technical innovations that went with it have
always been very highly rated at BPEG. Substantial progress in powder
making technology has been achieved in over 40 years. Some of the
outstanding milestones along the way were:
1950's Construction of rolling ball type mill (E type mill)
1960's Production of plate type mill
1970's Manufacture of rolling ball type mill (ZQM series)
1980 Start to manufacture of roller type mill (ZGM series)
1985 MPS mill license agreement with DEUTSCHE BABCOCK
1991 ZGM113 (MPS225), Output of mill 87.7t/h
1992 ZGM123, Output of mill 107.6t/h
1998 ZGM133, Output of mill 129.9t/h
Integrated CAD system and complex computer programs are being used
for product design. They accurately meet our customers' requirement.
New products are being developed in close co-operation with technical
colleges and institutes, such as, Tsinghua University, Huazhong
University of Science and Technology, as well as Thermal Power Research
Institute of China.
3. State-of-the-Art in Manufacture and Quality
High product quality is assured by State-of-the-Art and well-tended
facilities. In our workshops, the most advanced machine tools and
testing facilities are equipped with that ensure the acknowledged
quality of our products.
-Case hardening of bevel gear and spur gear.
-Gear machining with KLINGELNBERG Cutting Machine and MAAG
-Gear measuring with 3-coordinate measuring machine.
-Castings produced by production line with resin sand.
-Blanking line by CNC control.
Quality tests involve consistent monitoring throughout the manufacture
of all components, i.e., from raw materials and sub-assemblies to
the final product. This system ensures the availability and long
life of our products.
Our quality assurance system has been certified for many years.
The quality standard has been updated by ISO9001-2000 system, which,
in its overall approach, covers all functional areas of our company.
The respective certificate is only regranted after the system has
been vetted again by a neutral institution. We have received this
4. ZGM roller vertical mill
For all grades-ranging from hard lignite to anthracite. ZGM vertical
mills have a firmly established reputation for economy in the preparatory
processing of coal. They are used mainly in direct pulverized coal
firing systems or steam boilers in power plant. They are also used
for supplying pulverized fuel to industrial furnaces in steel works,
to rotary cement kilns in cement works and coal gasification plants.
4.2 Grinding Principle
A grinding table rotating round a vertical axis is driven by a
planetary gearbox. The grinding rollers, mounted in fixed positions,
are pressed down on the table under loading. The axes of the grinding
rollers are inclined at 12°„~15°„, in combination with the grinding
surface of the table they achieve optimum grinding effect, as experience
in actual plant operation shows.
The big grinding roller performs not only a purely rolling motion,
but also a sliding motion because the roller axis and grinding table
axis do not intersect in the plane of the table. The differential
speeds due to this configuration are a desirable feature, producing
an additional comminuting action by the shearing forces developed.
Large-diameter rollers can be arranged to run on a small surface
area of the grinding table. Bearing in mind that the mill throughput
rate increases in proportion to the square of the roller diameter.
The advantage of using big rollers will be evident.
4.3 Operation and design features
The raw coal is fed centrally from above through the classifier
mounted over the mill or from one side onto the rotating grinding
table. On passing under the rollers the coal is ground by their
action assisted by the pressure of the hydropneumatic spring loading
of the roller. When the roller travels over the bed of material
on the table, a system of loading frame and rods connected to the
working piston of a hydraulic cylinder causes oil to be displaced
from the space below the piston into hydropneunatic accumulators.
The pulverized coal is flung by centrifugal force into the zone
over the nozzle ring (louver air ring) surrounding the table. Where
the coal particles are swept upwards by the stream of hot air into
the classifier. The intimate contact with the hot air causes the
moisture contained in the coal to evaporate spontaneously, so that
the desired exit temperature of the pulverized product from the
mill (70°„~120°„) is already attained in the grinding chamber. The
oversize particles rejected by the classifier fall back onto the
grinding table for further size reduction. The fine particles, i.e.,
the finished product, are carried out of the classifier with the
The mill is driven by a special motor through a planetary gearbox.
A segmental thrust bearing in the gearbox absorbs the forces exerted
by the rollers. Before the mill drive is started, the grinding rollers
are lifted hydraulically off the grinding table. This action is
performed by oil pressure diverted from the cylinders exerting the
spring loading. In this way, the mill can be started, either empty
or filled with coal, with only a small starting torque, corresponding
to about 40 percent of the rated torque. The special motor that
can be started direct-on-line is adequate. No auxiliary drive is
4.4 Range of types
ZGM mills equipped with three rollers are used for coal grinding.
The increase of radius of grinding table is applied in mills with
a mean radius of grinding table from 650mm upwards.
4.5 Noise, Vibrations, Fire prevention
Industrial health and safety regulations lay down requirements
with regard to noise emission, mechanical vibrations and safety
of coal grinding mills.
In the power generating industry, the sound pressure level of a
mill system, measured at a distance of one meter from it, is not
allowed to exceed 85dB(A) under full load or part load operation
or under starting or stopping conditions. In addition, these are
regulations for limiting the vibration velocity of mill and building
foundations over the whole load range, as well as safety regulations
for the prevention of fires and explosions in coal grinding systems.
To meet these requirements, ZGM roller grinding mill incorporate
various constructional design and control engineering features:
-Extremely quiet running is obtained because each grinding roller
is individually mounted and restrained in the loading frame in combination
with gentle hydropneumatic spring loading.
-Vibrations are prevented by raising and lowering the rollers, timed
in co-ordination with the raw coal feed, on starting and stopping
-Movable clearing devices installed under the grinding table keep
the gas flow duct free from foreign bodies and material that settles
-As all the internal surfaces of the grinding chamber are vertical
or sloping, no deposits of coal can form on them.
4.6 Safety and plant layouts
Stored raw coal and pulverized coal may undergo self-ignition.
The explosion hazard associated with coal dust and pulverized coal
has been dealt with in many publications.
Active safety measures to protect coal-grinding plants can be applied.
Active measures against explosion hazard comprise operating the
system under inertized internal atmosphere (with or without a separately
introduced inert gas), avoidance of sources of ignition, and using
simple and clear plant layouts.
4.7 Rotary classifier
The rotary classifier is equipped with a bladed rotor. Product
fineness is governed mainly by the rotor speed, which can be varied
to suit requirements. This type of classifier is used in cases where
fineness corresponding to less than 10 percent retained on the 0.09mm
sieve is required or the product must have a narrow grain size distribution.
4.8 Coal feeding
The feeder for supplying the coal to the mill must ensure a satisfactory,
uniform and controllable flow of material.
There are dry coals with good flow properties, which can be extracted
from the bins by means of rotary gate or table feeder. For coals
with poor flow ability, however, continuous flow feeders with large
intake cross-sections are indispensable. The inlet arrangements
for delivering the material from the feeder into mill call for particular
attention. One solution is provided by the big vertical inlet pipe,
with a smooth lining to promote sliding of the feed material, passing
through the center of the classifier. In that case, the feeder and
bin must be mounted above the classifier. Considerable headroom
inside the building is saved by introducing the raw coal sideways
into the mill by means of screw feeder.
4.9 Control Behavior of ZGM Coal mills
ZGM mills can be controlled within the range from 25 percent to
100 percent of throughout capacity.
Central grinding plants are operated at a constant throughput rate,
which can be adapted to inter-mediate storage capacity for the pulverized
The mill throughout of direct-firing systems for boiler furnaces
and rotary kilns is governed by the fuel feed rate requirements
of the burners.
Control comprises three control loops:
-Raw coal feed
-Air flow rate through the mill
-Temperature in the classifier
The coal feed control loop is given its set point by the load controller
of the boiler. The feed rate is varied by adjustment of the speed
of the raw coal feeder.
In practice, when the coal feed rate to a direct-firing system
is varied, the rates of air flow though the mill is varied proportionally
between 70 percent and 100 percent. Tests have shown that continuous
adjustment of the airflow rate over the mill loading control range
considerably improves the control behavior of roller grinding mills.
In addition, and anticipatory lead can be applied to the fuel control
and air control action.
4.10 The aim is operational reliability
Besides the layout of the grinding plant, the machines with which
the system is equipped are a major factor governing its operational
reliability. The criteria applicable to the selection of all the
-Economy of the plant as a whole
-Mechanical dependability in continuous operation
-Efficient handling and conveying of the materials, avoiding the
formation to deposits of coal and coal dust.
-Easy maintenance and rapid remedying of faults
-Simple cleaning of the plant
Maintenance of ZGM roller grinding mills comprises:
-Changing the oil of the bearing in which the rollers are mounted,
changing the oil in the gearbox
-Replacement of worn parts
-Removing reject material (if this operation is not mechanized).
Changing the grinding elements can be accomplished simply and quickly
with special tools, via the hinged housing segment without disassembling
the housing or ducts°°